Electronic Design

Energy-Harvest Modules Provide Predictable Runtimes

Suppose you're designing a really low-power telemetry application— so low-power that you want to run it off free energy that you "harvest" from the environment. Let's say it's intended to monitor the vibration signatures of the wheel trucks in railroad boxcars and send data about those signatures to trackside collection points via a wireless link. Suppose further that you want to power the application from those same vibrations.

Advanced Linear Devices' ALD EH300 and EH301 EPAD energy-harvesting modules can accept energy from an assortment of widely available waste energy sources, such as vibration, light, chemical reactions, fluid and air flow, and environmental heat. The modules then store this energy to power conventional 1.8- and 5.0-V systems (see the figure).

The most thoughtful feature of these modules is that their datasheets tell you the minimum length of time a useable voltage will be available on their output terminals. That's a key design parameter. It lets the engineer writing the application code know the maximum number of clock cycles that a complete loop of wake-up, data acquisition, number-crunching, data output, and shutdown has to fit into. The ALD energy harvesting module may provide more time, but it will never provide less.

The modules are fairly simple. There is a growing number of options for the power source. In its examples, ALD uses piezoelectric fiber elements from Advanced Cerametrics. These elements are built using ACI's spun-piezoelectric Viscose Suspension Spinning Process (VSSP), which produces more rugged transducers than monolithic piezos.

The modules' unique elements include ALD's EPAD MOSFETs, which use precharged floating gates to precisely controlled threshold voltages. One EPAD MOSFET on the module input gates the voltage from the transducer that charges the module's storage capacitor from a trickle of electrons up to a preset maximum voltage. The second EPAD device controls the output, shutting off discharge when the voltage on the capacitor reaches a preset minimum.

The EH300 modules provide an output voltage that ranges from a high of 3.6 V to a low of 1.8 V. The output range on the EH301 modules is 5.2 to 3.1 V. So for a nominal 3.3- or 5-V rail, the output of the module is always within the standard operating voltage range of the microcontroller and associated circuitry selected for the application.

Given that voltage control, the capacitance on the module determines the energy available between VMAX and VMIN and therefore the minimum draw-down time at any constant current. The usable energy capacity of the EH300 is 4.6 mJ, which equates to 68 ms at 25 mA. For the "A" version of the EH300, the specs are 30 mJ, or 75 ms at 150 mA.

The 5.2 to 3.1-V EH301 also has a base version and an "A" version. The former provides 8.3 mJ, or 80-ms operating time at 25 mA. The latter provides 55 mJ, or 88 ms at 150 mA.

The ALD EH300 and EH301 EPAD are available now starting at $36.54 in quantities of 1000, with higher-volume prices (100,000 units) in the $20 range.

Advanced Linear Devices
Advanced Cerametrics

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